Syndiotactic Poly(methyl methacrylate) (sPMMA)−Polybutadiene (PBD)−sPMMA Triblock Copolymers:  Synthesis, Morphology, and Mechanical Properties

A series of syndiotactic poly(methyl methacrylate) (sPMMA)−polybutadiene (PBD)−sPMMA triblock copolymers, or MBM, have been successfully synthesized by using dilithium initiators (DLi's) based on the diadduct of tert -butyllithium ( t -BuLi) to either 1,3-bis(1-phenylethenyl)benzene (PEB) or m -diisopropenylbenzene ( m -DIB). The efficiency of these DLi's in building up MBM triblock copolymers has been compared under the same experimental conditions, i.e., in a cyclohexane/diethyl ether mixture for the butadiene polymerization at room temperature and in a cyclohexane/THF mixture for the MMA polymerization at −78 °C. Although all the synthesized copolymers show a monomodal, symmetric, and very narrow molecular weight distribution, the MBM copolymers synthesized with the m -DIB/ t -BuLi diadduct are pure triblocks and show a high tensile strength, in contrast to copolymers initiated by the PEB/ t -BuLi diadducts that are of a lower tensile strength and contaminated by MB diblock copolymers. Solvent cast films of MBM prepared with the m -DIB/ t -BuLi diadduct are two-phase materials as confirmed by DSC and dynamic mechanical analysis (DMA). Transmission electron microscopy (TEM) shows a spherical morphology at a low sPMMA content, that changes into a cylindrical and finally lamellar morphology upon increasing the sPMMA content. Phase separation is observed for MBM with M̄ n of the sPMMA blocks as low as 6000. Dependence of tensile strength on copolymer structure and sample preparation has been studied. The smaller sPMMA molecular weight, M̄ n (PMMA), required for high tensile strength depends on the PBD molecular weight, M̄ n (PBD), e.g. 12 000 for M n (PBD) = 36 000 and 6000 for M n (PBD) = 80 000. The upper M̄ n (PMMA) is ca. 20−25 000, whatever the M n (PBD). The optimum tensile strength is observed for M̄ n (PMMA) = 15 000, independently of M n (PBD) in the studied range. As a rule, the tensile strength tends to level off and the elongation at break starts to decrease when the sPMMA content is increased beyond 35 wt %. At a constant sPMMA content, M n (PBD) (>ca. 36 000) does not affect the ultimate tensile properties.